We have continued our study of the structure of chromatin, the complex of proteins and DNA found in eukaryotic nuclei. We have probed in detail the sensitivity of the adult Beta globin gene of chicken to a variety of nucleases. We find that in erythrocytes the entire 6 kilobase region containing this gene is uniformly about 10 fold more sensitive to digestion with pancreatic DNase than is bulk DNA or other specific gene sequences. We have also found and mapped in great detail a hypersensitive site in erythrocyte chromatin near the 5' end of this gene. This is the first detailed analysis of such a site. Since transcriptionally active chromatin undergoes changes in methylation of cytosine, principally at the sequence CpG, we have synthesized the polynucleotide poly(m5dc-dG).poly(m5dC-dG) in order to study the effects of methylation on DNA structure. We find that the methylated polymer can undergo transitions to a left-handed Z-like conformation under physiological salt conditions (unlike the unmethylated polymer, which requires very high salt concentrations). We have measured the helical repeat and other physical properties of this polymer.